Converter and control method thereof

ABSTRACT

A converter includes a transformer, a primary side switch, a load detection circuit, a state detection circuit and a control circuit. The transformer is configured to output a voltage to a load. The primary side switch is coupled to a primary winding and a primary ground terminal. The load detection circuit is configured to detect a load state of the load and output a load state signal. The state detection circuit is configured to detect a reference time point. The control circuit is configured to output a control signal to turn on or off the primary side switch. The control circuit further sets a blanking time according to the load state signal, such that the primary side switch is turned on when a drain-source voltage of the primary side switch is at a valley of the resonance after the blanking time starting from the reference time point.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of the U.S.application Ser. No. 16/246,539, filed Jan. 13, 2019, which claimspriority to China Application Serial Number 201810194211.1, filed Mar.9, 2018, all of which are herein incorporated by reference in theirentireties.

BACKGROUND Technical Field

The present disclosure relates to a converter, especially with regard toa flyback converter.

Description of Related Art

In recent years, switching power supply has been widely applied toportable mobile devices such as laptops, tablet computers, smart phones,and so on. The miniaturization, high efficiency, and high frequency arethe trend of switching power supply.

Wherein the flyback converter with QR control mode has been widely usedin the low power field, especially applied for the application withpower less than 100 W, because of simple circuit structure, low cost,and low switching loss with valley turning on.

However, the conventional QR control mode of flyback converter is notsuitable for the development trend of miniaturization and high switchingfrequency due to the switching loss increased rapidly with highswitching frequency. In order to decrease the switching loss, a newcontrol method of flyback converter is provided.

SUMMARY

One aspect of the present disclosure is provided a converter. Theconverter includes a transformer, a primary side switch, a secondaryside switch, a load detection circuit, a state detection circuit and acontrol circuit. The transformer includes a primary winding and asecondary winding. The primary side switch is electrically coupled tothe primary winding and a primary ground terminal. The secondary sideswitch is electrically coupled to the secondary winding and a load. Theload detection circuit is configured to detect a load state andcorrespondingly output a load state signal. The state detection circuitis configured to detect a reference time point. The control circuit isconfigured to output a control signal to turn on or turn off the primaryside switch. The control circuit is configured to set a blanking timeaccording to the load state signal, such that the primary side switch isturned on when a drain-source voltage of the primary side switch is at avalley of the resonance after the blanking time starting from thereference time point. The transformer further includes a primaryauxiliary winding. The state detection circuit is configured to detect atime point when a cross voltage of the primary auxiliary winding startsto oscillate, and sets the reference time point based on the time point.

Another aspect of the present disclosure is a control method of aconverter. The control method includes: detecting a load state by a loaddetection circuit and correspondingly outputting a load state signal;setting a blanking time by a control circuit according to the load statesignal; detecting a reference time point by a state detection circuit;outputting a control signal to a primary side switch of the converter bythe control circuit so as to turn on the primary side switch when adrain-source voltage of the primary side switch is at a valley of theresonance after the blanking time starting from the reference timepoint. Wherein the control method further includes: detecting a timepoint when a cross voltage of the a primary auxiliary winding of atransformer starts to oscillate; and setting the reference time pointbased on the time point.

Another aspect of the present disclosure is provided a converter. Theconverter includes a transformer, a primary side switch, a secondaryside switch, a load detection circuit, a state detection circuit and acontrol circuit. The transformer includes a primary winding and asecondary winding. The primary side switch is electrically coupled tothe primary winding and a primary ground terminal. The secondary sideswitch is electrically coupled to the secondary winding and a load. Theload detection circuit is configured to detect a load state andcorrespondingly output a load state signal. The state detection circuitis configured to detect a reference time point. The control circuit isconfigured to output a control signal to turn on or turn off the primaryside switch. The control circuit is configured to set a blanking timeaccording to the load state signal, such that the primary side switch isturned on when a drain-source voltage of the secondary side switch is ata peak value of a resonance after the blanking time starting from thereference time point. The state detection circuit is configured todetect a time point when the drain-source voltage of the secondary sideswitch starts to oscillate, and sets the reference time point based onthe time point.

Another aspect of the present disclosure is a control method of aconverter. The control method includes: detecting a load state by a loaddetection circuit and correspondingly outputting a load state signal;setting a blanking time by a control circuit according to the load statesignal; detecting a reference time point by a state detection circuit;and outputting a control signal to a primary side switch of theconverter by the control circuit so as to turn on the primary sideswitch when a drain-source voltage of the secondary side switch is at apeak value of a resonance after the blanking time starting from thereference time point; wherein the control method further includes:detecting the drain-source voltage of a secondary side switch of asecondary side rectifier circuit of the converter by the state detectioncircuit; and recording the reference time point based on a time pointwhen the drain-source voltage of the secondary side switch starts tooscillate.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the disclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure can be more fully understood by reading thefollowing detailed description of the embodiment, with reference made tothe accompanying drawings as follows:

FIG. 1 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 2 is waveforms of the first control signal Sc1, the second controlsignal Sc2, the primary side current Ip, the secondary side current Is,the drain-source voltage Vds1 of the primary side switch, and thedrain-source voltage Vds2 of the secondary side switch of the converterin some embodiments of the present disclosure.

FIG. 3 is a schematic diagram of the relation chart between the loadstate signal and the length of the blanking time in some embodiments ofthe present disclosure.

FIG. 4A is a schematic diagram of the waveforms of the heavy load insome embodiments of the present disclosure.

FIG. 4B is a schematic diagram of the waveforms of the medium load insome embodiments of the present disclosure.

FIG. 4C is a schematic diagram of the waveforms of the light load insome embodiments of the present disclosure.

FIG. 5 is a schematic diagram of voltage and current waveforms of theconverter in some embodiments of the present disclosure.

FIG. 6 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 7 is waveforms of the secondary side current Is, the drain-sourcevoltage Vds1 of the primary side switch, the cross voltage Vaux of theprimary auxiliary winding, trigger signal Sa and start signal TB_startin some embodiments of the present disclosure.

FIG. 8 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 9 is waveforms of the first control signal Sc1, the drain-sourcevoltage Vds1 of the primary side switch, the secondary side current Is,the second control signal Sc2, the drain-source voltage Vds2 of thesecondary side switch and turn on signal Vy in some embodiments of thepresent disclosure.

FIG. 10 is a flowchart illustrating a control method in some embodimentsof the present disclosure.

DETAILED DESCRIPTION

For the embodiments below is described in detail with the accompanyingdrawings, embodiments are not provided to limit the scope of the presentdisclosure. Moreover, the operation of the described structure is notfor limiting the order of implementation. Any device with equivalentfunctions that is produced from a structure formed by a recombination ofelements is all covered by the scope of the present disclosure. Drawingsare for the purpose of illustration only, and not plotted in accordancewith the original size.

It will be understood that when an element is referred to as being“connected to” or “coupled to”, it can be termed “electrically connectedto” or “electrically coupled to”, and it can be directly connected orcoupled to the other element or intervening elements. As used herein,the term “and/or” includes an associated listed items or any and allcombinations of more. In addition, although terms such as “first”,“second” are used to describe different elements, it should beunderstood that such words are used to distinguish elements oroperations which are described using the same terminology. Unlessotherwise stated, such words are not intended to imply any specificorder or sequence or to limit the scope of the present disclosure.

Referring to FIG. 1, the converter 100 is configured to convert an inputvoltage Vin received from an input voltage source into an output voltageVo. In some embodiments, the converter 100 may be a flyback converter,but the present disclosure is not limited thereto.

As shown in FIG. 1, the converter 100 includes transformer 110, aprimary side switch S1, a load detection circuit 120, a state detectioncircuit 130 and a control circuit 140. The transformer 110 includes aprimary winding M1 and a secondary winding M2. The transformer 110 isconfigured to transmit the received power from the primary winding M1 tothe secondary winding M2. Structurally, the first terminal of theprimary winding M1 is electrically coupled to the positive terminal ofthe input voltage Vin. The second terminal of the primary winding M1 ofthe transformer 110 is electrically coupled to the first terminal of theprimary side switch S1. The second terminal of the primary side switchS1 is electrically coupled to a primary ground terminal (or negativeterminal of input voltage Vin). In other words, the primary side switchS1 is electrically coupled between the primary winding M1 and theprimary ground terminal. The control terminal of the primary side switchS1 is configured to receive a first control signal Sc1 to turn on orturn off the primary side switch S1. For example, the primary sideswitch S1 turns on when the first control signal Sc1 has a first level(e.g., high level). Relatively, the primary side switch S1 turns offwhen the first control signal Sc1 has a second level (e.g., low level).

Further, the converter 100 includes a secondary side rectifier circuit.As shown in FIG. 1, the secondary side rectifier circuit includes asecondary side switch S2, and the secondary side switch S2 is connectedbetween the secondary winding M2 and load. Further, the secondary sideswitch S2 is electrically coupled to the first terminal of the secondarywinding M2 and the first terminal of the output capacitance Co. In someother embodiments, the secondary side switch S2 can be arranged betweenthe second terminal of the secondary winding M2 and the second terminalof the output capacitance Co. The control terminal of the secondary sideswitch S2 receives a second control signal Sc2 to control the secondaryside switch S2 on or off. Wherein, the secondary side switch S2 may beMOSFET, IGBT or GaN devices. In some other embodiments, the secondaryside switch S2 may be a diode or other components.

Specifically, when the primary side switch S1 is turned on, a primaryside current Ip flows through the primary winding M1 of the transformer110, and correspondingly stores the energy in the transformer 110. Atthis time, the polarity of the secondary winding M2 of the transformer110 is opposite to the polarity of the primary winding M1, and thesecondary side switch S2 is off. No current flows through the secondaryside switch S2, and no energy is transferred from the primary winding M1to the secondary winding M2. The energy received by the load is providedby the output capacitor Co.

Relatively, when the primary side switch S1 is turned off, the polarityof the windings will reverse. At this time, the secondary side switch S2conducts so as to the energy of the transformer 110 transfers to thesecondary winding M2 from the primary winding M1 and forms a secondaryside current Is. The secondary side current Is flows through thesecondary side switch S2, such that the energy stored in the transformer110 transmits to the load and output capacitance Co through thesecondary side switch S2.

When the energy of the transformer 110 is transferred to the load andthe output capacitance Co, the secondary side current Is is graduallydecreased. When the secondary side current Is drops to zero, theparasitic capacitance C1 of the primary side switch S1 will resonatewith magnetizing inductance Lm, resulting in corresponding oscillationof the drain-source voltage Vds1 of the primary side switch S1. Then,the primary side switch S1 of the converter turns on through the firstcontrol signal Sc1 again, so that the primary side current Ip flowsthrough the primary winding M1 to store energy to the transformer 110.Accordingly, by repeatedly controlling turn on or turn off of theprimary side switch S1 and the secondary side switch S2, the converter100 can convert the input voltage Vin into the output voltage Vo.

In order to decrease the switching loss, the optimal time to turn on theprimary side switch S1 is when the drain-source voltage Vds1 of theprimary side switch S1 at valley of the resonance. In the presentdisclosure, the state detection circuit 130 is configured to detect areference time point. The reference time point is corresponding to atime point when the secondary current Is in the secondary winding M2drops to zero. The load detection circuit 120 is configured to detectthe state of the load, and correspondingly outputs a load state signalVfb. The control circuit 140 confirms the present load state of theconverter 100 is in a light load state, a medium load state or a heavyload state according to the load state signal Vfb so as to set ablanking time. Then the control circuit 140 outputs a first controlsignal Sc1 to turn on the primary side switch S1 when the drain-sourcevoltage Vds of the primary side switch is at a valley of the resonanceafter the blanking time starting from the reference time point.

Accordingly, since the control circuit 140 sets the blanking timeaccording to the load state, and the blanking time is not affected bythe switching frequency of the primary side switch S1. The convertercontrol method of the present disclosure is compatible with theconverter 100 with any switching frequencies design.

According to one aspects of the present application, the converter 100includes a clamp circuit 150 which includes a clamp resistor R3, a clampcapacitance C3 and a diode D1. The clamp circuit 150 is parallel to theprimary winding M1 and configured to clamp the drain-source voltage Vds1of the primary side switch S1 when the primary side switch S1 is turnedoff.

Referring to FIG. 2, similar elements related to the embodiment of FIG.1 are assigned with the same reference numerals for betterunderstanding. For convenience and clarity, the waveforms of the firstcontrol signal Sc1, the second control signal Sc2, the primary sidecurrent Ip, the secondary side current Is, and the drain-source voltageVds1 of the primary side switch S1 of the converter 100 shown in FIG. 2will be described with the embodiments shown in FIG. 1, but not limitedthereto.

At the time point t0, the secondary side switch S2 is turned on. Duringthe time t0 to t1, the converter 100 is in the state of transferringenergy to the load from the transformer 110, and the secondary currentIs decreases gradually. At the time point t1, the secondary current Isdrops to zero, and the first control signal Sc1, the second controlsignal Sc2 both are keeping low level, and the secondary side switch S2is turned off. The drain-source voltage Vds1 of the primary side switchS1 starts to oscillate. Therefore, the time point t1 is the “referencetime point” described above.

During the time t0 to t1, if the load detection circuit 120 detects thatthe converter 100 is in heavy load, the control circuit 140 will set theblanking time equal to zero. When the control circuit 140 detects thatthe drain-source voltage Vds1 of the primary side switch starts tooscillate and at a valley of the resonance for the first time (timepoint t2), the control circuit 140 outputs the first control signal Sc1(e.g., becomes to high level) to turn on the primary side switch S1.During the time t2-t3, the first control signal Sc1 is in high level andthe primary side switch S1 is turned on so as to allow the primarycurrent Ip flows through the primary winding M1 and primary side switchS1. Therefore, the drain-source voltage Vds1 of the primary side switchS1 is zero.

At the time point t3, the first control signal Sc1 switches from highlevel to low level. Correspondingly, the primary side switch S1 isturned off and the primary current Ip becomes zero. During the timet3-t4, the secondary current Is flows through the secondary side switchS2. As the energy stored on the transformer 110 is transferred to theload, the secondary current Is will gradually decrease from its maximumvalue to zero.

In some embodiments, for example, if the load detection circuit 120detects that the converter 100 is in medium load. Starting from the timepoint t4, the control circuit 140 sets a blanking time. After theblanking time, when the drain-source voltage Vds1 of the primary sideswitch S1 is at a valley of the resonance again (e.g., the time point t5in FIG. 2), the control circuit 140 generates the first control signalSc1 to turn on the primary side switch S1 again. The above time pointst2-t5 may be considered as one of the working period of the converter100. By repeatedly controlling the primary side switch S1 and thesecondary side switch S2 to turn on or turn off, the converter 100 canconvert the input voltage Vin into the output voltage Vo and output itto the load.

Specifically, the converter 100 sets different length of the blankingtime according to the load state signal Vfb. Wherein, the length of theblanking time increases as the load state decreases. That is, there isnegative correlation between the blanking time and the magnitude of theload state signal. For example, the converter 100 may work in the heavyload state, the medium load state and the light load state. When theconverter 100 is in the heavy load state, the control circuit 140selects a heavy load time as the blanking time. When the converter 100is in the medium load state, the control circuit 140 select a mediumload time as blanking time. The medium load time is longer than theheavy load time. When the converter 100 is in light load state, thecontrol circuit 140 selects a light load time as the blanking time. Thelight load time is longer than the medium load time. When the converter100 is in very light load, the control circuit 140 generates a turn onsignal to turn on the primary side switch S1 after a longer blankingtime starting from the reference time point without considering thevalley.

Referring to FIG. 3, FIG. 3 is the relation chart between the load statesignal Vfb and the length of the blanking time in some embodiments ofthe present disclosure, wherein the horizontal axis is the load statesignal Vfb which represents the load state of the converter 100. Thevertical axis is the blanking time which represents the length ofblanking time that should be set. As shown in FIG. 3, the characteristicline of relation chart is like a ladder. There are multiplecorresponding critical values V10, V11, V21, V20-V60, V61 on thehorizontal axis and multiple corresponding blanking times TB1-TB6 on thevertical axis.

The control circuit 140 adjusts the length of blanking time along withthe trend of the ladder shaped relationship line. For example, as shownin FIG. 3, when the load state signal Vfb decrease to the critical valueV21, the control circuit 140 adjusts the blanking time from TB1 to TB2.When the load state signal Vfb increase to the critical value V20, thecontrol circuit 140 return the blanking time from TB2 to TB1.

The heavier load state (e.g., the load state signal Vfb become larger),the shorter blanking time. In other words, the lighter load state (e.g.,the load state signal Vfb becomes smaller), the longer blanking time. Inthis way, the control circuit 140 can adjust the length of the blankingtime, according to the magnitude of the load state signal Vfb.

Referring to FIG. 4A-4C, FIG. 4A-4C are waveforms of drain-sourcevoltage Vds1 of the primary side switch S1, blanking signal TBx, turn onsignal Vy, and the first control signal Sc1 of the converter 100 in the“heavy load state”, “medium load state” and “light load state”. As shownin FIG. 4A, the blanking time is zero in the heavy load state, so thatthe blanking signal TB0 in the control circuit 140 maintains to highlevel. When the control circuit 140 detects that the drain-sourcevoltage Vds1 of the primary side switch S1 is at a valley of theresonance, the control circuit 140 generates the first control signalSc1 to turn on the primary side switch S1. In some embodiments, thecontrol circuit 140 detects the drain-source voltage Vds1 of the primaryside switch S1 by a valley detection circuit 141. When detecting thatthe drain-source voltage Vds1 of the primary side switch S1 is at avalley of the resonance, the control circuit 140 generates a turn onsignal Vy to the control circuit 140.

Similarly, as shown in FIG. 4B, in the medium load state, the controlcircuit 140 sets the blanking signal TB2 to low level from the referencetime point, and during the time when the blanking signal TB2 is lowlevel, the turn on signal Vy does not work. After the blanking timestarting from the reference time point, the control circuit 140 sets theblanking signal TB2 to high level. At this time, when the valleydetection circuit 141 detects that the drain-source voltage Vds1 of theprimary side switch S1 is at a valley of the resonance, the controlcircuit 140 outputs the first control signal Sc1 (e.g., becomes to highlevel) to turn on the primary side switch S1. As shown in FIG. 4C, inthe very light load state, the control circuit 140 directly generatesthe turn on signal Sc1 to turn on the primary side switch S1 after alonger blanking time starting from the reference time point withoutconsidering the turn on signal Vy.

Referring to the FIG. 1-5, when the secondary current Is becomes zero,the drain-source voltage Vds1 of the primary side switch S1 starts tooscillate at the same time. Therefore, in some embodiments, the statedetection circuit 130 detects the drain-source voltage Vds1 of theprimary side switch S1 and records the time point when the drain-sourcevoltage Vds1 of the primary side switch S1 starts to oscillate as thereference time point.

According to one aspects of the present application, the state detectioncircuit 130 includes a sensing capacitor Cs and a comparator 131. Thefirst terminal of the sensing capacitor Cs is electrically coupled tothe primary winding M1 and the primary side switch S1. The secondterminal of the sensing capacitor Cs is electrically coupled to thefirst terminal of the comparator 131. The first terminal of thecomparator 131 is further electrically coupled to a voltage source V1through a resistor R1, and electrically coupled to a ground terminalthrough a resistor R2. The second terminal of the comparator 131 iselectrically coupled to a reference voltage Vref1. In some embodiments,the state detection circuit 130 further includes a signal processcircuit 132. The signal process circuit 132 is connected to the outputterminal of the comparator 131. When the drain-source voltage Vds1 ofthe primary side switch S1 starts to oscillate, the sensing capacitorgenerates corresponding voltage change and current change. The currentIa will flow through the sensing capacitor Cs. When the voltage Vadecreases and is less than the reference voltage Vref1, the comparator131 outputs a trigger signal Sa to the signal process circuit 132, andthe signal process circuit 132 outputs the start signal TB_start to thecontrol circuit 140 according to the trigger signal Sa.

Referring to FIG. 6, FIG. 6 is a schematic diagram of a converter 100 insome embodiments of the present disclosure. In FIG. 6, similar elementsrelated to the embodiment of FIG. 1 are assigned with the same referencenumerals for better understanding. The specific principles of similarelements have been described in detail in the previous paragraphs, itwill not be described herein.

As shown in FIG. 6, the transformer 110 further includes a primaryauxiliary winding M3 and the state detection circuit 130 includes acomparator 131. The two terminals of the primary auxiliary winding M3respectively connect to the first input terminal of the comparator 131and the primary ground terminal, and the second terminal of thecomparator 131 is connected to the primary ground terminal. In someembodiments, the state detection circuit 130 further includes a signalprocess circuit 132. The signal process circuit 132 is connected to theoutput terminal of the comparator 131. As shown in FIG. 7, when thesecondary current Is becomes zero, the drain-source voltage Vds1 of theprimary switch S1 and the cross voltage Vaux of the primary auxiliarywinding M3 start to oscillate at the same time. When the cross voltageVaux of the primary auxiliary winding M3 starts to oscillate and crosszero voltage, the comparator 131 outputs a trigger signal Sa to thesignal process circuit 132. Then, the signal process circuit 132 outputsthe start signal TB_start to the control circuit 140 to record thereference time point. The reference time point is the time point whenthe cross voltage Vaux cross zero voltage. In some embodiments, thesecond terminal of the comparator 131 is connected to a referencevoltage Vref2, when the cross voltage Vaux of the primary auxiliarywinding M3 starts to oscillate and cross the reference voltage Vref2,the comparator 131 outputs a trigger signal Sa to the signal processcircuit 132.

In some embodiments, the state detection circuit 130 is also configuredto detect the drain-source voltage Vds2 of the secondary side switch S2and records the time point when the drain-source voltage Vds2 of thesecondary side switch S2 starts to oscillate as the reference timepoint. Referring to FIG. 8 and FIG. 9, the converter 100 includes asignal process circuit 132, and the load detection circuit 120, thestate detection circuit 130 and the peak detection circuit 142 areintegrated in the signal process circuit 132. The signal process circuit132 is electrically coupled to the two terminals of the secondary sideswitch S2 so as to detect the drain-source voltage Vds2 of the secondaryside switch S2. The load detection circuit 120, the state detectioncircuit 130 and the peak detection circuit 142 may respectively outputcorresponding load state signal Vfb, start signal TB_start and turn onsignal Vy according to the drain-source voltage Vds2 of the secondaryside switch S2.

Further, the state detection circuit 130 is electrically coupled to thetwo terminals of the secondary side switch S2 to detect the drain-sourcevoltage Vds2 of the secondary side switch S2. As shown is FIG. 9, whenthe secondary current Is becomes zero, the drain-source voltage Vds2 ofthe secondary side switch S2 starts to oscillate correspondingly and theoscillating phase is opposite to the drain-source voltage Vds1 of theprimary side switch S1. By detecting the time when the drain-sourcevoltage Vds2 of the secondary side switch S2 starts to oscillate, thestate detection circuit 130 outputs the start signal TB_start throughthe signal processing circuit 132 to record the reference time point. Insome other embodiments, the state detection circuit 130 is electricallycoupled to the secondary side switch S2 so as to detect the time pointwhen the secondary current Is of the secondary winding becomes zero, andrecord the time point when the secondary current Is of the secondarywinding becomes zero as the reference time point.

The connection relationship and the specific structure of the loaddetection circuit 120 are not the limitations of the present disclosure.One skilled in the art can understand the configuration of the loaddetection circuit 120 and therefore will not be described here. In someembodiments, as shown in FIG. 8 and FIG. 9, the load detection circuit120 detects the negative peak value of the drain-source voltage Vds2 ofthe secondary side switch S2, and outputs the load state signal Vfbaccording to the negative peak value of the drain-source voltage Vds2 ofthe secondary side switch S2. Because the load state signal Vfb isproportional to the peak current Ipk of the primary side switch S1, andthe formula below is satisfied:

Vfb=K1×Rcs×Ipk

Isk=n×Ipk

Vds2min=Rds×Isk

In the above three formulae, K1 is a coefficient, Rcs is sense resistorto detect the peak current of the primary side switch, n is the turnratio of transformer 110, Rds is the on-resistance value of secondaryside switch S2. Isk is the peak value of the secondary current Is. Ipkis the peak value of primary current Ip. Vds2min is the negative peakvalue of the drain-source voltage Vds2 of the secondary side switch S2.According to these formulae, the relationship between Vds2min and loadstate signal Vfb can be obtained:

$V_{{ds}\; 2\min} = {\frac{n}{K_{1}}\frac{R_{ds}}{R_{cs}}V_{fb}}$

Accordingly, the load detection circuit 120 can output the load statesignal Vfb according to the negative peak value of the drain-sourcevoltage Vds2 of the secondary side switch S2.

As shown in FIG. 9, since the secondary side switch S2 and the primaryside switch S1 starts to oscillate at the same time, the time point whenthe drain-source voltage Vds2 of the secondary side switch S2 is at thepeak of resonance is the same as the time point when the drain-sourcevoltage Vds1 of the primary side switch S1 is at a valley. So thecontrol circuit 140 can detect whether the drain-source voltage Vds2 ofthe secondary side switch S2 is at the peak of the resonance through apeak detection circuit 142. The control circuit 140 outputs the firstcontrol signal Sc1 to turn on the primary side switch S1 when thedrain-source voltage of the secondary side switch S2 is at the peak ofthe resonance after the blanking time starting from the reference timepoint.

Referring to FIG. 10, FIG. 10 is a flowchart illustrating a controlmethod in some embodiments of the present disclosure. For ease andclarity of explanation, the following control method is described inconjunction with the embodiments shown in FIGS. 1, 6 and 8, but is notlimited thereto. Anyone who is familiar with this skill, within thespirit and scope of the present disclosure, can make various changes andretouching.

First, in step S01, the load detection circuit 120 is configured todetect the load state, and outputs the load state signal Vfbcorrespondingly. The load state signal Vfb is configured to indicate theoutput power. In some embodiments, as shown in FIG. 1, the loaddetection circuit 120 detects the voltage of the two terminals of theload. In some embodiments, as shown in FIG. 8, the load detectioncircuit 120 detects the drain-source voltage Vds2 of the secondary sideswitch S2 to calculate the load state signal Vfb.

In step S02, the control circuit 140 is configured to receive the loadstate signal Vfb, and sets the blanking time according to the load statesignal Vfb. The length of the blanking time can change with the loadstate, such as the heavy load state, the medium load state, light loadstate or every light load state.

In step S03, the state detection circuit 130 is configured to detect thereference time point. The reference time point is corresponding to thetime point when the secondary current Is of the secondary winding M2 ofthe transformer 110 drops to zero. In some embodiments, as shown FIG. 1,when the secondary current Is becomes zero, the drain-source voltageVds1 of the primary side switch S1 oscillates at the same time.Therefore, the state detection circuit 130 detects the drain-sourcevoltage Vds1 of the primary side switch S1, and record the time pointwhen the drain-source voltage Vds1 of the primary side switch S1 startsto oscillate as the reference time point. In some embodiments, as shownin FIG. 8, when the secondary current Is becomes zero, the drain-sourcevoltage Vds2 of the secondary side switch S2 oscillates at the sametime. Therefore, the state detection circuit 130 detects thedrain-source voltage Vds2 of the secondary side switch S2, and recordsthe time point when the drain-source voltage Vds2 of the secondary sideswitch S2 starts to oscillate as the reference time point.

In step S04, the control circuit 140 outputs the first control signalSc1 to the primary side switch S1 so as to turn on or turn off theprimary side switch S1, and the primary side switch S1 is turned on whenthe drain-source voltage Vds1 of the primary side switch S1 is at avalley after the blanking time starting from the reference time point.In some embodiments, as shown in FIG. 1, the control circuit 140 detectsthe time when the drain-source voltage Vds1 of the primary side switchS1 is at valley by the valley detection circuit 141. In someembodiments, as shown in FIG. 8 and FIG. 9, since the drain-sourcevoltage Vds2 of the secondary side switch S2 also oscillates when thesecondary current Is becomes to zero and the oscillating phase isopposite to the drain-source voltage Vds1 of the primary side switch S1,the control circuit 140 may detect the time point when the drain-sourcevoltage Vds2 of the secondary side switch S2 is at the peak of theresonance and generates the first control signal Sc1 to turn on theprimary side switch S1 after the blanking time starting from thereference time point.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentdisclosure without departing from the scope or spirit of the presentdisclosure. In view of the foregoing, it is intended that the presentdisclosure cover modifications and variations of this present disclosureprovided they fall within the scope of the following claims.

What is claimed is:
 1. A converter, comprising: a transformer comprisinga primary winding and a secondary winding; a primary side switchelectrically coupled to the primary winding and a primary groundterminal; a secondary side switch electrically coupled to the secondarywinding and a load; a load detection circuit configured to detect a loadstate and correspondingly output a load state signal; a state detectioncircuit configured to record a reference time point; and a controlcircuit configured to output a control signal to turn on or turn off theprimary side switch, wherein the control circuit is configured to set ablanking time according to the load state signal, such that the primaryside switch is turned on when a drain-sauce voltage of the primary sideswitch is at a valley of a resonance after the blanking time startingfrom the reference time point, wherein the transformer further comprisesa primary auxiliary winding, the state detection circuit is configuredto detect a time point when a cross voltage of the primary auxiliarywinding starts to oscillate, and sets the reference time point based onthe time point.
 2. The converter of claim 1, wherein the state detectioncircuit comprises a comparator; two terminals of the primary auxiliarywinding are respectively connected to a first input terminal of thecomparator and the primary ground terminal, when the cross voltage ofthe primary auxiliary winding starts to oscillate, the comparatorcorrespondingly outputs a start signal to the control circuit to recordthe reference time point.
 3. The converter of claim 2, wherein a secondterminal of the comparator is connected to the primary ground terminal,so that the comparator outputs the start signal to the control circuitto record the reference time point when the cross voltage of the primaryauxiliary winding starts to oscillate and cross zero voltage.
 4. Theconverter of claim 2, wherein a second terminal of the comparator isconnected to a reference voltage, so that the comparator outputs thestart signal to the control circuit to record the reference time pointwhen the cross voltage of the primary auxiliary winding starts tooscillate and cross the reference voltage.
 5. The converter of claim 1,wherein a length of the blanking time is negative correlation with amagnitude of the load state signal.
 6. The converter of claim 1, whereinthe load detection circuit is configured to detect a negative peak valueof a drain-source voltage of the secondary side switch, and outputs theload state signal according to the negative peak value of thedrain-source voltage of the secondary side switch.
 7. The converter ofclaim 1, wherein the control circuit detects whether a drain-sourcevoltage of the secondary side switch is at a peak value of a resonancethrough a peak detection circuit, and the primary side switch is turnedon when the drain-source voltage of the secondary side switch is at thepeak value of the resonance after the blanking time starting from thereference time point.
 8. A control method of a converter, comprising:detecting a load state by a load detection circuit and correspondinglyoutputting a load state signal; setting a blanking time by a controlcircuit according to the load state signal; detecting a reference timepoint by a state detection circuit; and outputting a control signal to aprimary side switch of the converter by the control circuit so as toturn on the primary side switch when a drain-source voltage of theprimary side switch is at a valley of a resonance after the blankingtime starting from the reference time point; wherein the control methodfurther comprises: detecting a time point when a cross voltage of the aprimary auxiliary winding of a transformer starts to oscillate; and,setting the reference time point based on the time point.
 9. The controlmethod of claim 8, further comprising: outputting a start signal to thecontrol circuit to record the reference time point by a comparator whendetecting an oscillation of the cross voltage of the primary auxiliarywinding of the transformer of the converter.
 10. The control method ofclaim 8, wherein the comparator outputs the start signal to the controlcircuit when the cross voltage of the primary auxiliary winding startsto oscillate and cross zero.
 11. The control method of claim 8, whereinthe comparator outputs the start signal to the control circuit when thecross voltage of the primary auxiliary winding starts to oscillate andcross a reference voltage.
 12. The control method of claim 8, wherein alength of the blanking time is negative correlation with a magnitude ofthe load state signal.
 13. A converter, comprising: a transformercomprising a primary winding and a secondary winding; a primary sideswitch electrically coupled to the primary winding and a primary groundterminal; a secondary side switch electrically coupled to the secondarywinding and a load; a load detection circuit configured to detect a loadstate and correspondingly output a load state signal; a state detectioncircuit configured to record a reference time point; and a controlcircuit configured to output a control signal to turn on or turn off theprimary side switch, wherein the control circuit is configured to set ablanking time according to the load state signal, such that the primaryside switch is turned on when a drain-source voltage of the secondaryside switch is at a peak value of a resonance after the blanking timestarting from the reference time point, wherein the state detectioncircuit is configured to detect a time point when the drain-sourcevoltage of the secondary side switch starts to oscillate, and sets thereference time point based on the time point.
 14. The converter of claim13, wherein the load detection circuit is configured to detect anegative peak value of the drain-source voltage of the secondary sideswitch, and outputs the load state signal according to the negative peakvalue of the drain-source voltage of the secondary side switch.
 15. Theconverter of claim 13, wherein the control circuit detects whether thedrain-source voltage of the secondary side switch is at the peak valueof the resonance through a peak detection circuit, and the primary sideswitch is turned on when the drain-source voltage of the secondary sideswitch is at the peak value of the resonance after the blanking timestarting from the reference time point.
 16. The converter of claim 13,wherein a length of the blanking time is negative correlation with amagnitude of the load state signal.
 17. A control method of a converter,comprising: detecting a load state by a load detection circuit andcorrespondingly outputting a load state signal; setting a blanking timeby a control circuit according to the load state signal; recording areference time point by a state detection circuit; and outputting acontrol signal to a primary side switch of the converter by the controlcircuit so as to turn on the primary side switch when a drain-sourcevoltage of the secondary side switch is at a peak value of a resonanceafter the blanking time starting from the reference time point; whereinthe control method further comprises: detecting the drain-source voltageof a secondary side switch of a secondary side rectifier circuit of theconverter by the state detection circuit; and recording the referencetime point based on a time point when the drain-source voltage of thesecondary side switch starts to oscillate.